ABS has developed Guidance Notes on Strength Assessment of Independent Type C Tanks, providing procedures for determining design loads on the Type C tanks and performing the strength evaluation of the tank and supporting structures.
As the report say, the strength of independent Type C cargo/fuel tanks and their supporting structures should be evaluated against yielding, buckling, and fatigue. For each failure mode, all standard and accidental load cases as well as the test load case should be considered and applied to the tank and supporting structures.
In structural analysis, design loads need to be determined for the design of a Type C cargo/fuel tank structure and its supports.
Typical loads for the design of a cryogenic pressure vessel include internal pressure loads due to tank pressurization, thermal loads due to temperature variations during the loading/ unloading of liquid cargo/fuel or initial cooling down period, deadweight of the tank and its contents, and reaction loads due to supports and attachments
ABS says.
In addition, the unique loads associated with operating in a marine environment, such as dynamic loads due to ship motions and sloshing loads, also need to be considered.
Furthermore, when the Type C tank is designed as a vacuum-insulated tank, the major components of this tank system include:
- An insulated inner tank
- Evacuated outer jacket
- Inside supports between two shells
- Outside supports.
The function of the outer jacket is mainly to maintain the vacuum as thermal insulation and to support the inner tank. The buckling strength of the outer jacket should be evaluated under the vacuum pressure.
The evaluation of inside supports between inner and outer vessels should be performed considering the load transfer due to the relative thermal contraction between the inner tank and the outer jacket.
Also, the vacuum pressure between the two vessels should be considered in the design pressure as well. As a result, the yielding, buckling, and fatigue evaluations should be considered for Type C vacuuminsulated tank.
Based on the most severe load condition which the vessel may experience during a service life and cyclic load condition, tank and supporting structures should be evaluated against yielding, buckling, and fatigue failures. Design load cases can be categorized as follows:
- Standard load cases (LC1 – LC4)
- Accidental load cases (LC5 – LC7)
- Tank test load case (LC8)
- Fatigue (Load Pairs 1 – 4 & 5)
During the service life, design load cases for the tank design should consider the appropriate combinations of different loads, such as internal pressure, external pressure, and sloshing loads. The internal pressure includes hydrostatic and hydrodynamic pressures, which are caused by both liquid acceleration and gravity in fully loaded condition. The design life of Type C liquefied gas tanks should not be less than the design life of the vessel
ABS mentions.